High refractive index glasses, multifocal corrective lenses containing same

ABSTRACT

The present invention relates to glasses of high refractive index having the following composition of oxides, expressed in percentages by weight:their use for the production of segments for multifocal corrective lenses; and multifocal corrective lenses incorporating at least one such segment in their structure.

This application is a 371 of PCT/US98/08352 filed Apr. 27, 1998 whichclaims the benefit of U.S. Ser. No. 60/051,538 filed Jul. 2, 1997.

FIELD OF THE INVENTION

The present invention relates to high refractive index inorganicglasses; said index being more specifically between 1.70 and 1.78. Saidglasses have particularly interesting uses as optical or ophthalmicglasses and are most particularly appropriate for manufacturing segmentswhich enter into the making of multifocal corrective lenses. Saidmultifocal corrective lenses which incorporate at least one segment in aglass of the invention make up an integral part of the presentinvention.

BACKGROUND OF THE INVENTION

A multifocal lens is constituted of a principal lens in glass and ofseveral segments in glass which are fused onto it. These segments have ahigher refractive index than that of the principal lens; this enablesobtaining areas of higher corrective power. Said segment(s) is (are)normally soldered onto the principal lens during a heat treatmentbringing the whole of the glasses to a temperature close to thesoftening point of the glass constituting the principal lens.Consequently, the segment glasses must have characteristics of viscosityclose to those of the glass of the principal lens. It must also haveclose thermal dilation characteristics, so that the resulting solderinghas a very low strain level.

The objective of the present invention was to obtain glasses ofrefractive index between 1.70 and 1.78, fusible particularly to anophthalmic glass manufactured by the Applicant Company (Corning S. A. ),of index 1.6, of Abbe number 41 and which has the following physicalcharacteristics:

thermal dilation coefficient: 100×10⁻⁷ K⁻¹ (between 25 and 300° C.),

softening point: 610° C.,

strain point: 460° C.,

annealing point: 490° C.

Said glass of the Applicant Company bears the Code-name 8008. It wasdescribed in U.S. Pat. No. 4,540,672.

It has been determined that, in order to be fusible to said glassCode-named 8008, the glasses of the invention must have a thermaldilation coefficient between about 90 and 110×10⁻⁷ K⁻¹ and a softeningpoint between about 570 and 620° C.

The technical problem to be solved within the context of the presentinvention has therefore been that of providing glasses having thespecifications below:

high refractive index: 1.70≦n≦1.78;

thermal dilation coefficient: between 90 and 110×10⁻⁷ K⁻¹ (between 25and 300° C.);

softening point: between about 570 and 620° C.

PRIOR ART

According to the prior art, numerous glasses having a refractive indexhigher than 1.7 and which are able to be used as segment glass arealready known. On the other hand, to the knowledge of the Applicant,there exists few of them which have the specifications indicated above;i.e. which combine a refractive index higher than 1.7 and properties ofdilation and viscosity such as stated above or close to those statedabove. Thus, the Applicant has picked out in said prior art:

U.S. Pat. No. 4,568,651 (HOYA) describing glasses having the followingcomposition of oxides expressed in percentages by weight:

SiO₂ 14-24 B₂O₃  6-14 Al₂O₃ 0-4 SiO₂ + B₂O₃ + Al₂O₃ 26-34 MgO  0-10 CaO 0-15 BaO  2-24 SrO  0-17 ZnO  0-12 BaO + SrO + CaO + ZnO 15-30 PbO38-50 ZrO₂  0-3.

These glasses have an index between 1.74 and 1.78, a thermal dilationbetween 82 and 95×10⁻⁷ K⁻¹, and a softening point between 560 and 640°C.

These properties are close to those which were sought after within thecontext of the invention. However, the composition of the glassesdescribed in said U.S. patent is completely different from that of theglasses of the present invention. These prior art glasses have a veryhigh PbO content and are devoid of alkalis and TiO₂;

the Japanese patent JP-A-61 146730 (NIPPON KOGAKU KK) describing glasseshaving the following composition of oxides, expressed in molarpercentages:

SiO₂  6-30 B₂O₃ 15-44 Li₂O  7-18 Na₂O + K₂O 0-3 MgO + CaO + SrO + BaO +ZnO  0-35 La₂O₃  3-10 ZrO₂ 3-5 Nb₂O₅  3-8.

These glasses have an index of 1.7 to 1.78, a dilation of 98 to 114×10⁻⁷K⁻¹ and a softening point of 548 and 588° C. However, here again, thecomposition range is very different to that which is claimed here; theseglasses contain neither TiO₂, nor PbO.

The U.S. Pat. No. 4,824,809 (SCHOTT) and the three Japanese patentsJP-A-8 26765, JP-A-8 26766 and JP-A-8 25957 (NIKON CORP.) may also bementioned which describe, respectively:

for the US patent, glasses having a refractive index of 1.73 to 1.78.The dilation and viscosity of said glasses are not specified. Theseglasses have contents in PbO and, in TiO₂ and in alkalis, respectivelymuch higher and much lower than those of the glasses of the invention;

for the JP patents, glasses having refractive indices and softeningpoints close to those of the glasses of the invention. The thermaldilation coefficients of said glasses are not specified. The compositionranges of said glasses are very different to that claimed here: theglasses in particular contain neither any PbO, nor any TiO₂.

In order to have the specifications such as stated above, the Applicanttherefore proposes an entirely original solution (glass compositions).

SUMMARY OF THE INVENTION

The Applicant proposes novel glasses having the following composition ofoxides, expressed in percentages by weight:

SiO₂ 27-36 B₂O₃ 0-9 Li₂O 1-5 Na₂O 1-7 K₂O 0-7 CaO 0-7 BaO  4-13 ZnO 0-8La₂O₃  4-15 PbO  4-27 TiO₂  7-18 ZrO₂ 0-9 Nb₂O₅ 0-8 with: Li₂O + Na₂O +K₂O  6-15 SiO₂ + TiO₂ + ZrO₂ 42-55 PbO + TiO₂ + ZrO₂ + Nb₂O₅  29-40.

The glasses of the invention are obtained essentially from oxidesidentified above, taken in the amount specified above. It is certainlynot excluded that other oxides intervene (vide infra) but, in any case,said other oxides do not intervene in significant amounts (always ≦5% ,generally ≦3% by weight, even ≦1% by weight) and do not have asignificant influence upon the properties sought after. They onlyintervene for adjustments.

DESCRIPTION OF THE INVENTION

The principal constituents of the glasses of the invention are SiO₂,PbO, TiO₂ and La₂O₃. Said glasses also contain Li₂O, Na₂O and BaO. Asregards the optional components (B₂O₃, K₂O, CaO, ZnO, ZrO₂ and Nb₂O₅),it is specified here, to all useful ends, that the minimal interveningamount from which they exert a significant effect is generally in theorder of 0.5%. Thus, the glasses of the invention cannot containoptional constituents or, if they do contain them, it is generally in aminimal amount of 0.5% (% by weight).

According to an advantageous variant of the first object of theinvention, the glasses have the following composition of oxidesexpressed in percentages by weight:

SiO₂ 28-35 B₂O₃ 2-9 Li₂O 2-5 Na₂O 2-6 K₂O 0-5 BaO  5-13 ZnO 0-7 La₂O₃ 4-14 PbO 10-27 TiO₂  9-15 ZrO₂ 0-4 (obviously with, as indicated above:Li₂O + Na₂O + K₂O  6-15 SiO₂ + TiO₂ + ZrO₂ 42-55 PbO + TiO₂ + ZrO₂ 29-40(no Nb₂O₅))

The high indices of the glasses of the invention are obtained bymaintaining the sum of the TiO₂, PbO, ZrO₂ and Nb₂O₅ contents between 29and 40%. The elements used preferably for obtaining this index are TiO₂and PbO. This is why the minimal content of these elements must be 7 and4% respectively. ZrO₂, in addition to its role upon the index, enablesimproving the chemical durability. The high cost of the startingmaterials which are sources of Nb₂O₅ renders this oxide less interestingthan the previous ones. Too high a TiO₂, ZrO₂ or Nb₂O₅ content causes adevitrification at low viscosity , while too high a PbO content isunfavorable for the density and the dispersion.

SiO₂, TiO₂ and ZrO₂ are the elements which most increase the softeningpoint. It has been observed that, in order to obtain a softening pointin the range desired, the sum of the SiO₂, TiO₂ and ZrO₂ contents shouldbe between 42 and 55% .

An SiO₂ content greater than 27% further allows guaranteeing a goodchemical durability while beyond 36% it is difficult to obtain thedesired index and dilation.

The alkali metal oxides in the proportions indicated above enableobtaining the dilation and the characteristics of viscosity soughtafter.

The alkaline-earth metal oxides as well as ZnO and La₂O₃ enableadjusting the index and the viscosity. In order to obtain the glasseshaving the highest liquidus viscosity, it has been observed that it wasbetter to use BaO rather than CaO.

B₂O₃ has the advantage of decreasing the viscosity whilst having littleinfluence upon the dilation.

In order to adjust the properties, as indicated above, other oxides suchas Al₂O₃, MgO, SrO can also be added. Their total content must not goover 5%.

It is also possible to add conventional finishing agents such as As₂O₃or Sb₂O₃, fluorides, bromides or chlorides with the proviso that theirtotal content does not exceed 1%.

If desired, bleaching agents can be added. It is also possible to colorthe glass by adding conventional coloring elements, e.g. transitionmetal oxides or rare earth oxides.

In a way that facilitates the manufacture and the forming of the glassesof the invention, glasses having a low tendency to devitrify and inparticular a high liquidus viscosity are sought after. The glassesdescribed in the preferred range are optimized to this end. Theirliquidus viscosity is greater than 3 Pa.s.

The manufacture of the glasses of the invention does not bring about anyparticular difficulty; it does not necessitate any unusual condition ormeasure. It is within the reach of the person skilled in the art.

The conventional starting materials, such as oxides, carbonates andnitrates, can be used for the preparation of the fillers to be melted.The usual precautions, as to the purity of said intervening startingmaterials, for obtaining optical glasses suffice (obviously if it isdesired to obtain glasses of optical quality).

According to another of its objects, the present invention relates tothe use of the glasses having the original compositions specified abovefor the production of segments, of high refractive index, of multifocalcorrective lenses. In other words, another objective of the presentinvention is multifocal corrective lenses which comprise, in a mannerknown per se, a principal lens in a glass of relatively low refractiveindex and at least one segment soldered to said principal lens in aglass of higher refractive index; said segment(s) beingcharacteristically in an original glass of the invention. Saidmultifocal corrective lenses of the invention generally comprise onesole segment of this type in their structure. The intervention ofseveral of said segments is however in no way excluded.

As indicated in the introduction of the present text, the glasses of theinvention have been most particularly developed to be fused with a glass(Code 8008) having the characteristics below:

thermal dilation coefficient: 100×10⁻⁷ K⁻¹ (between 25 and 300° C.);

softening point: 610° C.;

refractive index: 1.6.

Thus the invention, within the context of its second objective, relatesmost particularly to multifocal corrective lenses whose principal lensis in said glass (having the characteristics above) and whose segment(s)is (are) in a glass of the invention.

The invention is illustrated in a totally non-limiting manner byExamples 1 to 10 below. Examples 1 to 4 belong to the preferred field.

According to an operating method specified below, glasses of theinvention have been prepared whose compositions are given in the Tablebelow. Said compositions are expressed in percentages by weight.

Operating Method

In each case, 2,500 g of starting materials were melted for one hour at1,250° C. in a platinum crucible. The glass was then made into bars ofone centimeter thickness.

The annealing was carried out with a cooling rate of 60° C./hour.

This operating method was carried out on the laboratory scale. It iswell obvious that the glasses of the invention may be manufacturedindustrially by using conventional methods of melting and forming.

In said Table 1 below, the physical characteristics of the glassesobtained are specified.

The dilation coefficients were measured between 25 and 300° C. They areexpressed in multiples of 10⁻⁷ K⁻¹.

The softening point or Littleton point is the temperature at which theviscosity of the glass is 10^(6.5) Pa.s. (10^(7.5) poises).

TABLE 1 1 2 3 4 5 6 7 8 9 10 SiO₂ 32.6 33.1 32.1 33.1 33.3 33.1 33.131.6 31.3 35.8 B₂O₃ 5.9 5.9 5.9 5.9 6.4 5.9 2.4 4.4 4.1 6.1 Li₂O 4.0 4.04.0 4.0 4.0 4.0 4.0 4.0 4.0 3.9 Na₂O 4.2 4.7 3.7 5.2 3.7 4.7 4.7 4.7 4.76.0 K₂O 4.1 CaO 5.6 5.6 5.6 4.1 5.6 4.6 BaO 11.9 6.3 11.9 11.4 6.3 6.36.3 7.8 6.3 5.2 PbO 21.1 18.1 24.1 18.1 18.1 18.1 18.1 18.1 18.1 5.0 ZnO5.6 ZrO₂ 2.9 2.9 2.9 2.9 2.9 2.9 2.9 6.9 2.9 2.5 TiO₂ 12.0 14.0 10.014.0 14.3 14.0 9.0 9.0 16.6 15.8 Nb₂O₅ 6.5 La₂O₃ 5.1 5.1 5.1 5.1 5.1 5.113.6 9.1 6.1 4.2 As₂O₃ 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Li₂O +Na₂O + K₂O 8.2 8.7 7.7 9.2 7.7 8.7 8.7 8.7 8.7 14.0 SiO₂ + TiO₂ + ZrO₂47.5 50.0 45.0 50.0 50.5 50.0 45.0 47.5 50.8 54.1 PbO + TiO₂ + ZrO₂ +Nb₂O₅ 36.0 35.0 37.0 35.0 35.3 35.0 30.0 34.0 37.6 29.8 Refractive Index1.740 1.742 1.737 1.739 1.744 1.740 1.735 1.737 1.768 1.710 Dilationcoefficient 96 93 97 96 94 97 101 98 98 101 Softening point 582° C. 579°C. 574° C. 588° C. 606° C. 600° C. 604° C. 602° C. 602° C. 606° C.Devitrification Liquidus 910° C. 950° C. 910° C. 950° C. 1000° C. Liquidus Viscosity (Pa · s) 7 3.5 7 3.5 2.5

What is claimed is:
 1. Glass having a refractive index in the range of1.70-1.78 and having compositions expressed in percentages by weight onan oxide basis, that consist essentially of: SiO₂ 27-36 B₂O₃ 0-9 Li₂O1-5 Na₂O 1-7 K₂O 0-7 CaO 0-7 BaO  4-13 ZnO 0-8 La₂O₃  4-15 PbO  4-27TiO₂  7-18 ZrO₂ 0-9 Nb₂O₅ 0-8 with: Li₂O + Na₂O + K₂O  6-15 SiO₂ +TiO₂ + ZrO₂ 42-55 PbO + TiO₂ + ZrO₂ + Nb₂O₅ 29-40


2. Glasses according to claim 1 having the following composition ofoxides, expressed in percentages by weight, that consists essentiallyof: SiO₂ 28-35 B₂O₃ 2-9 Li₂O 2-5 Na₂O 2-6 K₂O 0-5 BaO  5-13 ZnO 0-7La₂O₃  4-14 PbO 10-27 TiO₂  9-15 ZrO₂  0-4.


3. A glass segment for a multifocal corrective lens, the lens having arefractive index in the range of 1.70-1.78 and having a composition inaccordance with claim
 1. 4. A glass segment in accordance with claim 3having a softening point of 570°-620° C. and a thermal dilationcoefficient of 90-110×110⁻⁷/° C.
 5. A glass segment in accordance withclaim 3 having a composition in accordance with claim
 2. 6. A multifocallens in accordance with claim 5 wherein the glass segment has a thermaldilation coefficient of 90-110×10⁻⁷/° C. and a softening point of570-620° C.
 7. A multifocal corrective lens comprising a principal glasslens having a refractive index lower than the refractive index of aglass segment incorporated in the principal glass lens, the glasssegment having a refractive index in the range of 1.70-1.78 and having acomposition in accordance with claim
 1. 8. A multifocal lens accordingto claim 7 characterized in that the glass of said principal lens hasthe following characteristics: thermal dilation coefficient: 100×10⁻⁷K⁻¹ (between 25 and 300° C.); softening point: 610° C.; refractive index1.6.